This invention relates generally to the field of electronic power regulation and control and more specifically to a power regulation apparatus and method for reducing energy consumption of a load, particularly inductive loads such as fluorescent lights and motors, while simultaneously shifting the power factor toward a leading reflected power factor, and maintaining low line harmonic distortion, low line crest factor, and low line noise.
A variety of AC power regulating circuits are known in the art in which AC power to a load (e.g., fluorescent lamps, motors, etc.) is regulated through control of an AC electronic switch (e.g., a thyristor) interconnecting the source of the AC power and the load. Many of these systems use some form of feedback control system to sense the load power. This information is then used to advance or retard the time relative to the initial zero crossing that the electronic switch is triggered into conduction during each half cycle of the AC power cycle. Thus, the device is turned on later in the cycle to reduce power and earlier to increase power. Once activated, the electronic switch typically turns off automatically thereby blocking current to the load when the load current reverses direction (i.e., at the zero crossing) as illustrated in FIG. 1A. For example, some prior art systems utilize a triac as an electronic switch because of its bi-directional conduction and high power characteristics. However, a triac only turns off when the current through the triac drops to zero. Thus, to decrease or increase the power to the load, the trigger phase angle is advanced or retarded and the portion of each half wave of AC input power which is applied to the load through the switch is thereby decreased or increased.
Power regulation of this type results in conduction occurring primarily during the later part of each half cycle of the AC power. This tends to cause an inductive (lagging) power factor, generates harmonic distortion and noise spikes reflected into the power line and causes a high crest factor. Such lagging power factors decrease power line efficiency (i.e., increase power line current for a given load power consumption) and frequently results in increased electric utility rates to the user. In addition, because current does not flow from the AC source during the time the electric switch is turned off, substantial harmonic distortion and noise is reflected into the power line which can interfere with the operation of sensitive electronic equipment.
In another type of prior art power regulation circuit, an electronic switch is turned on and off several times during each half cycle to control the current to the load, as illustrated in FIG. 1B. Inductive energy is dispitated by switching a short circuit across the load when the electronic switch is turned off. This type of circuit also can produce or aggravate an inductive power factor, and generates harmonic distortion, noise, and a high crest factor on the AC power line. Thus, EMI and RFI filtering is required.
In either type of prior art regulating circuit, current from the AC power line to the load is interrupted during a substantial portion of each AC half cycle which can result in large surge currents. This large surge current can cause ballast temperature to rise excessively causing early failure or actual breakdown with acrid smoke generation. In addition, when used to power lighting loads, such as fluorescent lights, at a reduced power level to conserve power these circuits cause a large reduction in light output.
It is accordingly an object of the present invention to provide a novel, economic and reliable method and apparatus for AC power regulation which permits reduced power consumption while providing a leading power factor and minimizing the reflected harmonic distortion, noise spikes and crest factor on the AC power line.
It is another object of the invention to provide a novel method and apparatus for AC power regulation which is self-adjusting for a wide range of loads.
It is another object of the invention to provide a novel method and apparatus for AC power regulation which provides a 25% reduced power consumption for fluorescent and other ballasted lighting loads without excessive light intensity loss while providing a leading power factor.
It is another object of the invention to provide a novel method and apparatus for AC power regulation utilizing a triac and a large parallel energy transfer capacitor substantially greater than 1 .mu.f in which current continues to flow through the load via the capacitor during the time the triac is off so that current flows during substantially all of the AC cycle and a substantial portion of the power to the load is provided by current flowing through the capacitor. It is another object of the invention to provide a novel method and apparatus for AC power regulation utilizing a triac and a parallel switched energy transfer capacitor bank, wherein the triac is switched on shortly after the zero crossing of each half wave of the AC cycle and is switched off when an adequate power level is reached substantially before the next zero crossing to provide a leading power factor.
Briefly according to one embodiment of the invention there is provided an AC power regulation system for controlling power to a load having an input for coupling to an AC power source for supplying AC power having zero current crossing points at the end of each half cycle and wherein the system includes an output for coupling to the load. The system includes a triggerable switch coupled between the input and the output, a capacitor bank for supplying a energy transfer capacitance coupled in parallel with the triggerable switch, a circuit for generating power sense signals responsive to the load power, means for triggering the triggerable switch into a state of conduction, and means for switching the triggerable switch into a non-conductive state. Control circuitry is provided, coupled to the means for triggering and the means for switching, for generating control signals responsive to the power sense signals to control the means for triggering and the means for switching such that the electronic triggerable switch is triggered to a conduction state at a selected turn on-time after each zero crossing, and switched back to the non-conducting state at a selected turn-off time before the next zero crossing. The selected turn-off time is chosen to obtain a desired power level and to improve the reflected power factor responsive to power sense signals. In another embodiment, the capacitor bank comprises a bank of switchable capacitors and the control circuitry comprises circuitry for generating control signals to switch the switchable capacitors to obtain a selected capacitance responsive to the power sense signals.
In one embodiment of the invention a monitor circuit is coupled across the triggerable switch to generate a differential voltage signal responsive to the voltage across the triggerable switch. A controller circuit operates to trigger the triggerable switch into a conductive state responsive to the differential voltage signal. In addition, the means for switching the triggerable switch into a nonconducting state may switch the triggerable switch by shunting current around the triggerable switch for a short period of time.